Transparent plastics have been widely used as a core material in optical and transparent display industries. In particular, transparent plastics such as PC (polycarbonate) or PMMA (polymethyl methacrylate) have been applied in displays, optical lens, transparent boards of large buildings, and automotive industries as a lightweight alternative to conventional glass due to its advantageous properties of high light transmittance, shatter-resistance and high-refractive index. However, these plastic resins have a drawback of low abrasion resistance, since they have lower surface hardness than glass. In order to overcome this drawback, it is rising as an important issue to develop a hard-coating technology for improving the surface hardness of plastics.
Materials used in hard-coating technologies resin are largely divided into organic, inorganic, and organic-inorganic hybrid materials. Organic materials such as acryl, urethane, melamine, etc. have advantages of organic materials such as flexibility and processability, but they have low surface hardness. In contrast, silicon-based inorganic materials have the properties of high surface hardness and transparency, but they have low flexibility and processability. Since hard-coating technology requires the advantages of both of these materials, using an organic-inorganic hybrid material has been attracting more attention than using either one of them. However, even though many studies have been actively made to integrate the benefits of both organic and inorganic materials into the hard-coating technology, they are still unsatisfactory.
In the conventional technologies, Japanese Patent Laid-open Publication No. 2006-063244 discloses a resin composition for hard coating, which is composed of colloidal silica surface-treated with a silane coupling agent having a reactive (meth)acrylate group in its molecule, a monomer having one reactive (meth)acrylate group in its molecule or a polymer prepared by polymerization of this monomer, a bifunctional (meth)acrylate, a tri- or higher multi-functional (meth)acrylate, a leveling agent, and a photo-polymerization initiator. However, photo-radical polymerization of acrylate is sensitive to oxygen, unlike polymerization of alicyclic epoxy group. Thus, when it is intended to obtain a cured hard-coating article using a photo-radical polymerization of acrylate, inert gas atmosphere should be maintained, which is regarded as a disadvantage.
Meanwhile, US Patent Laid-open Publication No. 2012-0034450 discloses a surface protection film, which can be obtained by mixing an ionizing radiation curable resin, a matting agent, an ultraviolet ray absorbing agent, and inorganic fine particles subjected to hydrophobization treatment to thereby prepare a resin, and then curing the resin. However, physical mixing of the ionizing radiation curable resin with the inorganic fine particles may decrease dispersibility and cause aggregation of inorganic fine particles. Transmittance is also decreased due to light scattering caused by the interface between the resin and the inorganic fine particles. Therefore, this film is not suitable for optical protection films.
Accordingly, it is expected that development of hard-coating materials with easy processability of organic materials and high light transmittance and surface hardness of inorganic materials will be an essential technology for a wide range of applications of plastics.